Readable Physical Storage Replica and Standby Database System

1. A computer-implemented method, comprising: a replica data system replicating changes made to data units at a primary data system to data unit replicas of the data units maintained at said replica system; and while replicating said changes to said data unit replicas: maintaining a latest-readable time; and performing read operations on data unit replicas maintained at said replica system consistent with said latest-readable time; wherein each of the above steps is performed by one or more computing devices.

2. The method as recited in claim 1 wherein at least one of said replica data system database and said primary data system comprise a multi-node system.

3. The method as recited in claim 1 wherein said latest-readable time is maintained based on which change descriptions for said primary data system have been applied to said data unit replicas of said replica system.

4. The method of claim 1 , wherein said data units are disk sectors.

5. The method of claim 1 , wherein said data units are data blocks storing records of a database.

6. A computer-implemented method, comprising: a standby database system replicating changes made to data blocks at a source database system to replicas of said data blocks at said standby database system; and while replicating said changes to said data blocks: receiving queries issued to said standby database system; and computing said queries based on data read from said data blocks; wherein each of the above steps is performed by one or more computing devices.

7. The method as recited in claim 6 wherein one or both of said standby database and said primary database systems comprise a multi-node system.

8. The method as recited in claim 6 , further comprising: maintaining a latest-readable query time; wherein said computing step comprises computing said queries consistent with transactions committed on said primary database system.

9. The method as recited in claim 8 wherein said latest-readable query time is maintained based on which redo records for said primary database have been applied to said data blocks of said standby database.

10. The method as recited in claim 8 wherein said latest-readable query time is maintained based on a plurality of transactions, wherein the complete redo records of the plurality of transactions have been applied.

11. The method as recited in claim 8 , further comprising: maintaining a certain cache of data items at the standby database system; and invalidating one or more data items in said certain cache based on changes to the latest-readable query time.

12. The method as recited in claim 11 wherein: the primary database system generating a redo log record indicating that a particular data item of said data items has changed at the primary database system; said standby database system invalidating said one or more data items based one said redo log record.

13. A computer-implemented method, comprising: a standby database system applying redo records that record changes to data blocks at a primary database system to replicas of said data blocks at said standby database system; in response to applying all redo records for a particular logical time, advancing an established logical time for computing queries to at least said particular logical time; and while replicating said changes to said data blocks: receiving a query issued to said standby database system; and computing results for said query that are consistent with said established logical time based on data read from said data blocks; wherein each of the above steps is performed by one or more computing devices.

14. The method as recited in claim 13 , wherein computing results includes said standby database system deferring completion of computing results until determining that undo records associated with said established logical time or before are received by said standby database system.

15. The method as recited in claim 13 wherein said standby database is a multi-node system database system.

16. The method as recited in claim 15 wherein said standby database system issues global locks on data blocks being modified.

17. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 1 .

18. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 2 .

19. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 3 .

20. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 4 .

21. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 5 .

22. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 6 .

23. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 7 .

24. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 8 .

25. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 9 .

26. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 10 .

27. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 11 .

28. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 12 .

29. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 13 .

30. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 14 .

31. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 15 .

32. A computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the method recited in claim 16 .